Hot water tank



Jan. 30, 1968 INCHES PENETRATION PER MONTH R. G. MURDOCK 3,366,263

HOT WATER TANK Filed July 5, 1964 F|G.l FIG.2

22 6 .v,.; \20 g a Q "I"II .OIS- FIG.3

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INVENTIOR o 1 ROBERT G.MURDOCK Ti/C RATIO ATTOB EY United States Patent M 3,366,263 HOT WATER TANK Robert G. Murdock, Muuhall, Pa., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., :1 corporation of Pennsylvania Filed July 15, 1964, Ser. No. 382,897 4 Claims. (Cl. 220-5) This invention relates to hot water storage tanks, and in particular to hot water storage tanks formed of stainless steel.

It is well known that hot water tanks have been utilized throughout the United States which vary in their construction from that of a plain galvanized tank which was formed and riveted together to the more sophisticated modern tanks which employ either a stone-like or a glasslike lining. Various forms of energy are utilized to provide the required heat. Competitive considerations have required this area of industry to look to providing a hot water storage tank with prolonged life. Quite naturally,

the manufacturers of hot water storage tanks investigated, inter alia, the use of stainless steel as a material for such tanks. The initial venture into this field was totally unsuccessful, since the material selected from which to form the hot water tanks was an austenitic stainless steel of the AISI 300 Series. This material failed in service since, in the fabrication of the hot water storage tank, the austenitic stainless steel was cold Worked and the component parts were welded to form the storage tank. The failure was diagnosed as stress corrosion. While it was possible to form the austenitic stainless steels and weld the component parts into the form of a hot water storage tank and thereafter heat treat the same in the fabrication of hot water storage tanks, this proved to be uneconomical and not a real solution to the problem.

Further attempts were made by changing the material from the austenitic type stainless steel to that of the ferritic type. Noteworthy in this respect, it was found that Type 430, when employed as such, was free from stress corrosion but suffered from other difficulties. In particu-v lar, when Type 430 was welded, part of the material in the heat affected zone of the weld was heated to a sufficiently high temperature that the austenitic phase was formed. Upon the subsequent cooling of this material, the cooling rate was sufficiently fast that a martensitic component was formed, thereby making the welds exceedingly brittle. This again necessitated both preand posttreatment of the welded tank, which again proved unsatisfactory because of the scaling problems involved as well as the added cost of this treatment.

In order to alleviate these difiiculties it was conceived that if the martensitic constituent could be eliminated from the welded area, the brittle weld would thereby be eliminated and no preor post-heat treatment would be necessary. To this end the composition described hereinafter was derived for this use.

An object of this invention is to provide a hot water storage tank which is formed from a terrific stainless steel.

A further object of the present invention is to provide a hot water storage tank which may be manufactured employing conventional fusion welding techniques without the need of a preor post-Weld heat treatment.

A further object of the present invention is to provide a hot water storage tank which is formed from a ferritic stainless steel containing titanium, the titanium being present in an amount of from ten times the carbon content minimum and up to 1.25% titanium.

A more specific object of the present invention is to provide a stainless steel hot water storage tank having a convex bottom and a convex top, all of welded construction, the hot water storage tank being formed from a 3,356,253 Patented Jan. 30, 1968 steel containing, as essential alloying components, chromium and titanium, the titanium being preferably present in an amount ranging between from about 12 to about 15 times the carbon content contained within the stainless steel.

Other objects of the present invention will become apparent when taken in conjunction with the following description and the drawings in which:

FIGURE 1 is a schematic view in section illustrating a prior art hot water storage tank;

FIG. 2 is a schematic view, in section, showing a hot water storage tank of the construction contemplated by the present invention; and

FIG. 3 is a plot of the corrosion rate versus the titanium to carbon ratio of the Huey test.

Referring now to the drawings, and to FIG. 1 in particular, the prior art hot Water storage tank is shown generally at 10 and comprises a vertically extending cylindrical side wall 12, a concave head 14 and a concave bottom 16. Usually, the vertically extending cylindrical side wall is formed from steel plate which is roll-formed and then seam welded as at 17. The tank is thereafter assembled by welding the head 14 and the bottom 16 to the cylindrical side wall 12 as at 18, in any suitable manner, for example, resistance welding or fusion welding. As set forth hereinbefore, when austenitic stainless steel was employed in the fashion of the construction of the hot water tank as shown in FIG. 1, failure occurred by stress corrosion. In addition it was found that the concave construction of the bottom 16 resulted in a crevice 20 between the vertically extending cylindrical side wall 12 and the bottom 16 in which the mineral deposits, shown generally at 22, collected. These mineral deposits were exceedingly difficult to remove, and upon the repeated application of heat they seriously interfered with the heat transfer to the Water contained in the hot water storage tank, and in addition the settling of these mineral deposits resulted in a corrosive action at the crevice 20. While the substitution of AISI Type 430 stainless steel eliminated the failures due to stress corrosion, the welded joints 18 resulted in brittleness occurring thereby failing to solve the problem.

In order to alleviate these conditions, it was believed necessary to remove all susceptibility to intergranular corrosion which would normally occur when the base metal is heated to a given temperature during the welding operation. Thus it was concluded that the sensitization temperature had to be raised in order to minimize the susceptibility of this material to integranular corrosion. It was then conceived that by restricting the composition as set forth hereinafter, the susceptibility to intergranular corrosion would be minimized. In general, it has been found that the hot water tanks must be formed of a steel having a composition within the ranges given hereinafter in order to minimize the susceptibility of these steels to intergranular corrosion when a welding treatment is employed. Table I, listed hereinafter, gives the general, as well as the preferred, range of composition.

In the stainless steel having the general composition set forth in Table I, it is desirable to limit the carbon content to 0.07% maximum, although the preferred carbon content is less than about 0.05%. The lower carbon content is desired from the standpoint that a cleaner steel 1 to determine the corrosion resistance of the steel and which serve as a basis for predicting the behavior of said steel in an application for a hot water tank employing an all-welded construction.

TABLE IL-GHEMICAL ANALYSES Element RV-7l.1 RV-712 RV-7l3 2C042 38850 38664 RV-7l4 RV-715 38653 045 040 050 044 045 050 046 050 042 38 39 42 40 4G 43 40 38 4U 006 008 007 008 015 O14 006 006 011 011 010 010 003 012 010 009 010 009 .48 .44 .45 .45 .46 .30 .26 .42 .46

can be employed, less titanium will be required, and the over-all chemistry can be more closely controlled. The chromium content is permitted to vary between 17.75% and 18.75%. The primary function of the chromium is to provide the required degree of corrosion and oxidation resistance, and at the same time suppress the gamma loop so as to provide a substantially ferritic microstructure in the steel in finished form. It is also for this reason that the nickel content of the present steel is limited to 0.5% maximum in order to fully suppress the austenite formation during heat treatment, thereby eliminating the formation of any martensitic constituent, either during cooling from the heat treatment or in any welding operation. It is also preferred to limit the aluminum content to 0.15% maximum. While the aluminum is used to deoxidize and positive additions are made to the bath to form a limealumina slag, nonetheless the aluminum content must be limited to .15 maximum in order to obtain the required control of the titanium content. The titanium content of the steel is highly critical and must be limited to within the narrow ranges set forth hereinbefore. In this respect it has been found that the minimum titanium present must be at least ten times the carbon content yet the maximum titanium may not exceed 1.25%. The minimum titanium is set so as to insure freedom from intergranular corrosion, as may be determined by the Huey test and as will be explained more fully hereinafter. The maximum, of course, is dictated by material cleanliness as well as obvious economies in melting and in the recovery of the titanium component. The balance of the composition is essentially iron with the usual incidental impurities and usual amounts of other components normally found in The steels set forth in Table II have a composition which falls generally within the range set forth hereinbefore in Table I and contain, as the variable, the ratio of the titanium to carbon. These steels in sheet form were subjected to the nitric-hydrofluoric acid test which consists of boiling the test coupon for three half-hour periods in a solution consisting of 15% by volume nitric acid and 3% by volume hydrofluoric acid. The test coupon which is subjected to this test consists of a piece of metal measuring one inch by two inches which was sheared in half and thereafter welded longitudinally, employing the wellknown heliarc method, and having no filler metal added. It was significant that Heats RV-7ll, RV-7l2 and RV- 713, which have a titanium to carbon ratio varying between 5.8 and 9.4 demonstrated intergranular attack in the heat-affected zones of the welds; however, when the titanium to carbon ratio was increased to an amount in excess of 10 to 1, as shown in Heats 20642, 38850, 38 864, RV-7l4, RV-7l5 and 38653, very little or no preferential attack occurred in the weld areas. While the foregoing test was essentially qualitative in that the specimen either passed or failed the test, nonetheless it served to indicate that a minimum titanium content of at least ten times the carbon content was highly critical in these steels.

Based on the foregoing results, the steel was next subjected to the so-called Huey test which consists of utilizing duplicate heliarc welded test coupons and immersing the same in boiling nitric acid for five consecutive 48- hour periods. The following Table III lists the results of these tests, calculated on the basis of inches penetration per month.

TABLE III.-HUEY TEST CORROSION RATES (BOILING 65% NITRIC ACID) Corrosion Rate (Inch Penetration Per Month) Heat Ti/C 1st Period 2nd Period 3rd Period 4th Period 5th Period Average RV-711 5. 8 0124 0227 0256 0202 0122 .0220 .0248 0107 RV-712 7. 6 0020 0071 0098 0124 0078 0021 0078 0101 0120 0080 RV 713 9. 4 0012 0026 0091 0138 0081 0012 0025 0090 0133 0079 20642 0014 0030 0028 0053 0033 0016 0028 0027 0054 0033 RV-714 12. 1 0014 0023 0040 0057 0041 0014 0022 0036 0052 0037 RV-715 13. 7 0015 0024 0042 0061 0046 0015 0027 0047 006G 0050 the commercial manufacture of these steels, for example, manganese within the range of between 20% and .50% silicon within the range between .20% and .50% and about .03% maximum phosphorus, sulfur and the like.

In order to more clearly demonstrate the advantages of the steel of the present composition, reference is dirooted to Table II which contains a listing of the chemical composition as well as the titanium to carbon ratios of a series of heats which were made and tested in order These test results are plotted in the accompanying FIG. 3 and clearly demonstrates that until the ratio of the titanium to carbon reaches about 10 to 1, the steel having the composition set forth hereinbefore is subject to a high rate of intergranular corrosion in the weld area. On the other hand, where the steel has a titanium to carbon ratio in excess of about 10:1 the corrosion rate is materially decreased.

The foregoing clearly demonstrates that on the basis of laboratory tests, the steel employed in the hot water storage tanks of the present invention possesses a marked freedom from intergnanular corrosion where the titanium content is maintained within the critical range. As a result thereof, heliarc welding, as well as any other form of fusion welding, may be employed onautomatic equipment in order to manufacture the hot water tank of the present invention. In this respect, attention is respectfully directed to FIG. 2 which illustrates a hot water tank which is shown generally at 30. This hot Water tank is formed of a vertically extending sidewall 32 fabricated from the steel having the composition set forth hereinbefore in Table I and which has been roll-formed and butt welded as at 34. Since the steel which is employed in the hot water tank of the present invention is resistant to intergranular corrosion, and is balanced in such a way that no martensite is formed upon welding it now becomes possible to utilize a convex head 36 and a convex base 38 and join the same to the vertically extending sidewall 32 by any of the fusion Welding techniques, for example heliarc welding as at 40. As a result, the hot water storage tank as thus fabricated eliminates the crevice problem as set forth in the prior art, and thereby makes the removal of any precipitated minerals quite easy in the tank of the present construction. Moreover, since the steel is essentially ferritic, it is not subject to stress corrosion, and with the critical titanium content the steel of the present invention does not demonstrate any marked susceptibility to sensitization or martensite formation resulting from welding. Since the normal fusion welding techniques can be employed in joining these materials, no special skills nor equipment are required in the practice of the present invention.

I claim:

1. A stainless steel hot water tank characterized by having a convex bottom and top, an all-welded construction, and a composition consisting of up to 0.07% carbon, from 0.2% to 0.5% manganese, from 0.2% to 0.5% silicon, from 17.75% to 18.75% chromium, up to 0.5% nickel, up to 0.15% aluminum, titanium in an amount ranging from times the carbon content up to 1.25%, and the balance essentially iron with incidental impurities.

2. A hot water storage tank having a vertically extending cylindrical side wall formed from welded sheet, and a convex bottom and a convex top each of which is welded to the vertically extending side wall, said hot water storage tank being fabricated from a ferritic stainless steel having a composition consisting essentially of up to 0.07% carbon, from 0.2% to 0.5% manganese, from 0.2% to 0.5% silicon, from about 17.75% to 18.75% chromium, up to 0.5% nickel, up to 0.15% aluminum, titanium in an amount ranging *from 10 times the carbon content up to 1.25%, and the balance essentially iron with incidental impurities.

3. A hot water storage tank having a vertically extending cylindrical side wall formed from welded sheet, and a convex bottom and a convex top each of which are welded to the vertically extending side wall, said hot water storage tank being fabricated from a ferritic stainless steel having a composition consisting essentially of up to 0.07% carbon, from 0.20% to 0.5% manganese, 0.2% to 0.5% silicon, from 17.75% to 18.75% chromium, up to 0.5 nickel, up to 0.15% aluminum, titanium in an amount ranging from 12 to 15 times the carbon content, and the balance essentially iron with incidental impurities.

4. A hot water storage tank having a vertically extending cylindrical side wall formed from welded sheet, and a convex bottom and a convex top each of which is welded to the vertically extending side wall, said hot water storage tank being fabricated from a ferritic stain less steel having a composition consisting essentially of up to 0.05% carbon, from 0.2% to 0.5% manganese, from 0.2% to 0.5% silicon, from 17.75% to 18.75% chromium, up to 0.5% nickel, up to 0.15% aluminum, titanium in an amount ranging from 12 to 15 times the carbon content, and the balance essentially iron with incidental impurities.

References Cited OTHER REFERENCES Republic Enduro Stainless Steels, 1951, published by Republic Steel Corp., pp. 37-39, Cleveland, Ohio.

Titanium in Iron and Steel, by Comstock, 1955, pp. 202-210, published by John Wiley and Sons.

Metals Handbook, 8th edition, volume I, 1961, pp. 546,

547, and 550, published by the American Society for Metals.

DAVID L. RECK, Primary Examiner. THERON E. CONDON, Examiner. I. M. CASKIE, P. WEINSTEIN, Assistant Examiners. 

1. A STAINLESS STEEL HOT WATER TANK CHARACTERIZED BY HAVING A CONVEX BOTTOM AND TOP, AN ALL-WELDED CONSTRUCTION, AND A COMPOSITION CONSISTING OF UP TO 0.07% CARBON, FROM 0.2% TO 0.5% MANAGANESE, FROM 0.2% TO 0.5% SILICON FROM 17.75% TO 18.75% CHROMIUM, UP TO 0.5% NICKEL, UP TO 0.15% ALUMINUM, TITANIUM IN AN AMOUNT RANGING FROM 10 TIMES THE CARBON CONTENT UP TO 1.25%, AND THE BALANCE ESSENTIALLY IRON WITH INCIDENTAL IMPURITIES. 